Abstract
SUMMARY A petrographic and geochemical survey of the Waterswallows Sill has shown it to be an olivine microphyric dolerite of a transitional type with tholeiitic affinities. The cores from 86 boreholes indicate a saucer-shaped intrusion 80 m thick with a coarse-grained upper unit and a fine-grained columnar lower unit. Eight new K-Ar isotopic age determinations gave a spread of ages from 160 to 280 my. These results are examined knowing that smectite (nontronite and saponite) alteration, and not chlorite alteration as previously reported, is present. The variable loss of argon related to the partial devitrification of the primary interstitial glass may explain these analyses. They do not indicate the age of emplacement of the Sill.
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